1. Field of the Invention
The present invention relates to an apparatus for introducing samples into a mass spectrometer, and more particularly to an apparatus for introducing samples into an inductively coupled plasma source mass spectrometer, thereby to measure the amount of uranium or thorium contained in semiconductor materials.
2. Description of the Related Art
If semiconductor material for manufacturing a memory (e.g., a dynamic memory) contains uranium (U) or thorium (Th), the radioactive element will emits rays as it undergoes spontaneous decay, inevitably causing software errors in the cells of the memory. In order to make a high-speed memory of a high integration density and a great storage capacity, the semiconductor material (e.g., packaging material and chip material) preferably contains as little U or Th as possible.
Among the known methods of measuring the content of U or Th in semiconductor materials are:
(1) Inductively coupled plasma emission spectrometry PA1 (2) Fluorescence spectrometry PA1 (3) Radio-activation analysis
The first two methods cannot measure an extremely small amount of U or Th contained in the samples. Besides, they cannot be used in practice where the samples have been subjected to complex chemical pre-treatment. These methods inevitably require a long time to analyze the samples. The last method, i.e., radio-activation analysis, is generally employed to measure the content of U or Th in semiconductor materials, but is not practical since it needs the assistance of a nuclear reactor.
Recently, an inductively coupled plasma source mass spectrometer has been used in an attempt to analyze U or Th, instead of any of the three methods mentioned above. This new method is disclosed in Rober S. Houk et al. Inductively Coupled Argon Plasma as an Ion Source for Mass Spectrometric Determination of Trace Elements, Anal. Chem., Vol. 52, pp 2283-2289, 1980, Alan R. Date et al., Plasma Source Mass Sepectrometry Using an Inductively coupled plasma and High Resolution Quadrupole Mass Filter, Vol. 106, 1255-1267, 1981 and U.S. Pat. No. 4,501,965, Donald J. Douglas. As these publications teach, the inductively coupled plasma mass spectrometry is carried out in the following way. First, a dissolved sample is made misty by a nebulizer. Next, the misty sample is introduced into the inductively coupled plasma, and changed into excited ions. These ions are mass-separated by means of a quadruple mass filter, whereby the content of U or Th in the sample is measured by electron multiplier.
The inductively coupled plasma mass spectrometry can provide more accurate results than the inductively coupled plasma emission spectrometry or fluorescence spectrometry. However, this method is disadvantageous in two respects. First, the samples cannot be introduced into the plasma with high efficiency. Second, the
detection accuracy is limited to 10.sup.-11 g to 10.sup.-12 g., and the method cannot practically apply to a small sample such as thin semiconductor film.
It has been proposed that an apparatus, which vaporizes samples, thereby to introduce the samples into a inductively coupled plasma mass spectrometer with an increased efficiency, be mounted on the mass spectrometer. In such an apparatus, a sample is placed on the heat-generating plate of a heater. The sample is heated gradually by the heater in an inert gas flow. The sample is heated, and the target element, U or Th, vaporizes. The heat-generating plate is made of a material having a high-melting point, such as graphite, tantalum, or tungsten.
When the plate is made of graphite, U or Th contained in the vaporized sample reacts with graphite, inevitably forming carbide. Consequently, the efficiency of ionization of U or Th decreases, or U or Th remains in the heat-generating plate to cause so-called "memory effect." Either insufficient ionization or the memory effect greatly reduces the accuracy of measuring the U content or the Th content.
On the other hand, when the plate is made of tantalum or tungsten, the U or Th, which is an impurity contained in the metal in a very small amount, is detected along with the U or Th contained in the sample. In this case, it is difficult to analyze the U or Th contained in the sample when the U or Th content of the sample is less than the U or Th content of the metal.